Dissolved organic carbon (DOC) released from fish and other animal waste in saltwater aquariums is neither biodegradable nor easily removed by mechanical or biological filters. Accumulated DOC causes the level of ammonia, nitrites, nitrates, and the potential for unwanted micro algae formation to increase which may be detrimental to the health of the animals populating the aquarium. Moreover, high DOC levels produce an undesirable yellow coloring of the water.
Foam fractionators, also known as protein skimmers, can remove the yellow colouring, greatly reduce toxins and raise overall redox levels in the aquarium to ensure long term, stable conditions for all inhabitants. Protein skimmers function by the adherence of DOC on the surface of fine air bubbles rising in a protein skimmer column. DOC's are essentially surfactants. A surfactant molecule has one site manifesting affinity for water molecules and another site manifesting affinity for air molecules. Thus, rising bubbles in a protein skimmer column, drag away DOC molecules from the coherent body of water. The entrained DOC molecules accumulate as froth on the water surface where it can be manually collected and discarded.
Two main types of protein skimmers are popular with sea water aquarium hobbyists. These can generally be classified in two categories namely the "co-current" type and "counter-current" type. The terms "co-current" and "counter-current" denote the direction of flow of the air bubbles, which have a propensity to rise in water, relative to the direction of water flow. When the air bubbles and the water both travel in the same direction, a protein skimmer is said to be of the "co-current" type, while when the air bubbles and the water travel in opposite directions, the protein skimmer is characterised as a "counter-current" type device. The basic, and least efficient type, is the co-current skimmer. Such a skimmer usually comprises a column and a wooden air diffuser located at the bottom of the column. An air supply conduit feeds air to the diffuser. The water inlet is at the bottom of the column and the outlet is, in some cases, the same as the inlet. In use, the air bubbles stream generated by the diffuser causes water to rise in the column. This pumping action produces a continuous water circulation in the column to expose, over time, the entire water content of the aquarium to the cleansing effect of the air bubble stream.
A known disadvantage of such protein skimmers is the comparatively short residence time of the air bubbles in the water. This reduces the efficiency of the skimmer since the rate of impurities removal depends on the length of time the air bubbles remain in the contaminated body of water.
An improved type of protein skimmer is the counter-current type which comprises a column and an air diffuser located at the bottom of this column. Contrary to the basic co-current type, the water is forced, by a supplemental air lift tube, to descend within the skimmer column against the upward flow of air bubbles resulting in a counter-current action. This counter-current flow greatly increases the air/water exposure time within the column. The result is greater skimmer efficiency in comparison to the co-current type. However, such a skimmer is more expensive to manufacture, requires dual feed air units and generally produces salt deposits which are not aesthetically pleasing.